fix: support optional iss claim for anonymous credential issuance

[nanderstabel]

Description of change

This PR updates the credential issuance flow to correctly handle an optional iss (issuer) claim in the Proof contained in the Credential Request, aligning with the OpenID4VCI specification.

Links to any relevant issues

• https://openid.net/specs/openid-4-verifiable-credential-issuance-1_0-15.html#section-8.2.1.1-2.2.2.1

How the change has been tested

• Manually tested against UniMe
• Added the with_anonymous_access test case to the test_credential_endpoint test.

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have successfully tested this change in a docker environment

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
..._rest/src/issuance/credential_issuer/credential.rs	97.33% <100.00%> (ø)
agent_event_publisher_http/src/lib.rs	73.91% <100.00%> (ø)
agent_issuance/src/credential/aggregate.rs	89.19% <100.00%> (+0.03%)	⬆️
agent_issuance/src/offer/aggregate.rs	81.89% <100.00%> (-0.39%)	⬇️
agent_issuance/src/offer/event.rs	50.00% <ø> (ø)
agent_issuance/src/offer/views/mod.rs	88.88% <100.00%> (ø)

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